The present invention relates to a new product in the form of a cloth comprising asbestos fibers, and to a method of producing said cloth and particularly in the form of a microporous membrane used as a diaphragm in electrolysis cells.
It is known that the diaphragm of an electrolysis cell behaves like a porous medium and permits, on the one hand, the passage of current with a low ohmic loss and, on the other hand, the uniform flow of the electrolyte from one compartment to another. This results in a set of mechanical, electrical and hydraulic conditions which are the more critical the more it is required to operate, in modern electrolysis cells, with a high-density current, apart from tolerating prohibitive ohmic losses.
The required qualities are quite contradictory. From the mechanical point of view the diaphragm must in fact have a definite and permanent geometry and it must be uniform as regards size and texture. It is necessary to avoid the phenomenon of swelling of the diaphragm, but at the same time the diaphragm must permit the release of the gases which are sometimes generated within the diaphragm.
From the electrical point of view, the diaphragm should have good relative resistance. By the term "relative resistance" is meant the quotient of the resistance of a medium constituted by the diaphragm soaked in electrolyte in relation to the resistance of the medium constituted solely by the same electrolyte.
It has been observed that this relative resistance is related not only with the porosity of the diaphragm, but also with the shape of the ducts through which the electrolyte flows. Finally, it is desirable to avoid the phenomena of diffusion of one medium into the other through the membrane, and particularly in the case of the electrolysis of a sodium chloride solution, it is necessary to prevent the flow of OH.sup.- ions in the direction opposite to that of the stream of liquid, which flow of OH.sup.- ions is responsible for the formation of chlorate and therefore for a drop in yield of electrolytic products. The effect of this disadvantage can be reduced by increasing the thickness of the diaphragm and by reducing its porosity, but the drop in voltage in the diaphragm is then increased.
Finally, from the point of view of hydraulics, the permeability of a diaphragm should be such that the loss of pressure is low. This permeability is a function of the size of the pores, but for the above-mentioned reasons excessively large pore-diameters cannot be tolerated.
A final requirement that arises is that of continued reliability with time. Present-day technology in fact tends more and more towards the use of cells having a relatively lengthy service life. In this context, the diaphragms used are expected fully to retain all their properties with the passage of time. The difficulty in obtaining this set of quite contradictory conditions explains the large number of proposals for solving these problems that have been put forward. Thus, it was proposed many years ago to make diaphragms mainly of asbestos fibers. Such diaphragms are obtained from a dispersion of asbestos in water. These diaphragms have proved to be particularly suitable as what are known as deposited diaphragms, that is to say diaphragms formed on the cathode itself, and this technique has itself resulted from the technological requirements imposed on cells using cathodes arranged in a finger-like formation.
However, developments in the art have led, on the one hand, to the design of other types of cells, particularly the filter-pass type, and on the other hand, to an increase in current densities for reasons of efficiency and increased yield.
This has brought two consequences: first, deposited diaphragms have given way to prefabricated diaphragms since diaphragms obtained by depositing asbestos have proved to be inadequate for high current densities, i.e., in excess of 15 amperes per square decimeter. Furthermore, it is known that the deposition of asbestos fibers can only lead to structures, the porosity of which is difficult to control, said structures also suffering from the disadvantages of non-consolidated structures, namely:
1. swelling during electrolysis, this requiring a substantially large interpolar distance; PA1 2. difficulties in obtaining thin deposits which permit a low ohmic loss; and PA1 3. an unstable condition of the diaphragm which, after commencement of electrolysis and stabilization, render it difficult to deal with breakdowns during operations and to effect repairs of replacements in situ. PA1 1. Providing flat diaphragms only, either because the use of calendering or pressing makes any other shapes impossible, or that the initial suspensions, in particular when they are coagulated, do not have sufficient properties to permit homogeneous deposits on supports of complex shape. PA1 1. in a first stage, asbestos fibers and at least one solid particulate filler or charge substance are mixed, in the dry state; PA1 2. in a second stage, at least one latex is added while the mixture is being subjected to slow malaxation; and PA1 3. a cloth is then formed by any known forming or shaping method. PA1 about 10 to 100 parts of solid particulate filler or charge substance PA1 about 1 to 100 parts of polymeric resin latex PA1 about 1 to 20 parts of water. PA1 10 to 100 parts of at least one particulate filler or charge substance PA1 1 to 100 parts of at least one polymeric resin latex PA1 1 to 20 parts of water PA1 0.5 to 2 parts of a plasticizing agent;
It is for these reasons that over the last few years the trend has been toward the use of diaphragms constituted by a plastics micro-porous membrane based on a polymer, usually polytetrafluoroethylene, which is resistant to the electrolytic medium.
Mixed solutions have also been proposed, for example in French Pat. No. 2,123,514, according to which there is provided a homogeneous suspension of asbestos fibers and carrier substances, such as bentonite, etc., this suspension being mixed with an acid-resistant latex.
Numerous other disclosures are to be found relating to prior art diaphragms. Mention may be made to the following patents which employ techniques of compression preforming followed by fritting, or techniques of coagulation of the mixture or depositing of this mixture on a support.
Thus, French Pat. No. 1,491,033 of Aug. 31, 1966, describes a process for manufacturing a porous diaphragm which consists in the sequence of: (1) mixing a solid additive in particulate form into an aqueous dispersion of polytetrafluoroethylene in the presence of particulate inorganic fillers, (2) then coagulating the dispersion, (3) placing the resulting coagulum in sheet form, and (4) finally removing the solid particulate additive from the sheet. The additive consists of starch or calcium carbonate and is removed at the end of the operation by immersing the resultant sheet in hydrochloric acid to dissolve the additive. The particulate inorganic fillers which are suitable are barium sulfate, titanium dioxide or powdered asbestos. They are used in proportions of between 40 and 70% of the weight of the polytetrafluoroethylene contained in the dispersion.
British Pat. No. 943,624 of Dec. 14, 1961, proposes a method of producing a filter material which consists in mixing polytetrafluoroethylene in powder form with an eliminatable powdered material, subjecting the mixture to preforming under high pressure, and then sintering the resultant shape at a temperature which does not affect the polymer, then powdered material being eliminated either by volatilization at the sintering temperature or by the addition of solvents in which it is solubilized.
German application No. 2,140,714 of Aug. 13, 1971 describes a process of manufacturing diaphragms having a base of inorganic fibers, particularly asbestos, which are bonded by a fluorinated resin. The membrane can be obtained by impregnating a paper or fabric, or else produced by the introduction of fibers into the resin suspension and shaping in accordance with a paper-making method. The sintering is then effected under elevated pressure.
All of these foregoing prior art techniques, however, have a number of drawbacks, namely:
2. Difficulties, in the case of membranes rich in polytetrafluoroethylene, in producing membranes of satisfactory mechanical properties (permitting large flow) and of good wettability.
3. Low percentage of voids is permitted in the diaphragm structure. In order to obtain good mechanical properties and excellent conservation of the cohesion during electrolysis, the quantities of pore-forming agents used are zero or low, namely, 200-300%, or less, by weight of material. Under these circumstances, the performances in the electrolysis of sodium chloride are not truly of interest -- rather large ohmic drop or low Faraday yield, resulting from the reduced porosporosity of the diaphragm.
Other prior art is also less than satisfactory. British Pat. No. 1,160,084, published July 30, 1969, discloses membranes and diaphragms produced from a matrix of a fluorocarbon polymer and a combustible fibrous substrate, such as of cellulose, which can be burned out of the matrix. The resulting product is porous in nature, due to the voids left by the burning of the cellulose. According to the patent asbestos in the diaphragm is to be avoided.
British Pat. No. 1,063,244, published Mar. 30, 1967, describes a porous medium which is unsuitable for use in electrolysis cells. It is comprised of a porous base, such as of paper, having fibers, such as of asbestos, adhered to the surface, with the aid of a polymeric binder.
Bachot et al. United States patent application, Ser. No. 469,808, filed May 14, 1974, discloses a method which consists in forming an asbestos suspension in the presence of a surface-active agent and in adding to this suspension the latex of the fluoric resin and the pore-forming material. Although such a method gives good results and, in particular, enables a large quantity of pore-forming agent to be introduced, it still requires a certain proportion of asbestos and the use of a wet process.
Diaphragms having a high proportion of cavities have also been proposed, these being obtained by adding a surface-active agent to the asbestos suspension. Unfortunately, despite the improvements thus obtained, there is still considerable difficulty in controlling the formation of the coagulum, on which the qualities, particularly the electrolytic qualities, of the diaphragms depend.
In particular, this difficulty stems, on the one hand from the nature of the polymer used, which almost necessarily has to be polytetrafluoroethylene and which has to be converted by the use of heavy compression and which, during this conversion, has the troublesome tendency to retain occluded gases, and on the other hand, from the need to use carrier substances having a fine and well-defined particle-size, if it is desired to obtain a satisfactory suspension.
It is, accordingly, an object of the present invention to provide more desirable cloths to be used as diaphragms for electrolytic cells.
It is another object of the present invention to provide processes for the preparation of semi-permeable membranes having superior properties.
Other objects will be apparent to those skilled in the art from the present description.